In an electro-photographic copier using Carlson process, after uniformly charging the surface of the photoconductive element, an electrostatic latent image is formed by imagewise exposure, and then the electrostatic latent image is developed with toner. The developed toner image is made to be a visual image by being transfer to a recording sheet and fixed thereon.
Thereafter, the photoconductive element is subject to removal of toner adhered to the surface thereof, discharging, and cleaning and, thus, is repeatedly used for a long period.
Therefore, the electro-photoconductive element is required to have improved electro-photoconductive properties such as charging property, sensitivity and reduced dark decay. Also, such physical properties as durability, abrasion-proof, moisture-proof under repetitive use and durability against ozon generated during corna discharge or against ultra-violet rays used for exposure, etc. are required to be good.
Up to now, as a photoconductive element for electrophotography, sensitive elements which comprises photoconductive layer containing, as its principal component, selenium, zinc oxide, cadmium sulfide, etc. have widely been used.
In the other hand, use of organic compounds as a photoconductive material has been actively studied and developed. For example, Japanese Patent Examined Publication No. 10496/1975 refers to organic photoconductive element containing in the photoconductive layer a poly-N-vinycarbazole and a 2,4,7-trinitro-9-fluorenone.
But this photoconductive element is not always satisfactory in sensitivity and durability. In order to avoid such disadvantage, an atempt to develop an organic photoconductive element having enhanced sensitivity and durability has been made, wherein charge generating function and charge transporting function are separately assigned to different materials in the photoconductive layer. Like this in the function-separated electrophotographic element, compounds having respective functions can be chosen from wide variety of compounds and, thus, production of a photoconductive element having optional properties becomes relatively easy.
Recently, quite a few compounds which are useful as a charge generating material for function-separated type photoconductive element have been proposed. As an example, in which an inorganic substance is used, anorphous selenium which is disclosed in Japanese Patent Examined Publication No. 16198/1969 may be mentioned. This is used in combination with an organic charge transporting material.
Many electro-photoconductive elements which uses organic dye or pigment as a charge generation material have been proposed. For example, photoconductive element having a photoconductive layer which contains bis-azo compound are disclosed in Japanese Patent Publication Open to Public Inspection No. 37543/1972, 22834/1980, 79632/1979 and 116040/1981.
However, in electro-photographic process especially, repeated using causes reduction of charging property because of the exposure time accumulatively increases to ozon and active materials.